Listed only by Irwin, who says: A Diapason or Diapason-Flute hybrid of 5-1/3' on the manuals, and 10-2/3' on the pedals, belonging to the sub-unison series of stop-pitches. It is formed from open metal or wood pipes varying in loudness according to the 16' manual and 32' pedal stops it is meant to augment. It sounds the important third harmonic of this series on manuals and pedals, and speaks at the fifth above the key depressed. Note that the only examples we know of are at Twelfth pitch rather than the Fifth pitch specified by Irwin.
Quinte Ouverte 2-2/3', Grand Orgue; St. Etienne du Mont, Paris, France.